The present invention relates generally to a method of improving both caustic and high-fired properties of magnesia. Specifically, the invention is directed to improving the compactability of caustic magnesia and the density of high-fired magnesia.
High-fired magnesia (MgO), known and hereinafter referred to as sintered periclase, forms the principal constituent of magnesia brick. To increase its refractory effectiveness it is desirable to maximize its density. High density of sintered periclase is promoted by high and uniform green density of the precursor caustic compacts. Further, production costs of periclase can be reduced by more efficient compaction and lowered calcining temperatures for the precursor caustics.
Properties of caustics prepared according to prior art show a very strong dependence on the caustic calcining temperature. Generally, reasonable compaction is achieved only for high calcining temperatures between 900.degree.-1000.degree. C. These high temperatures are costly both in terms of energy and kiln maintenance. Further, because small variations in calcining temperature may cause considerable variation in caustic properties, briquetting efficiency and green density of the briquettes are often either too low or too variable. Therefore, it has been necessary to carefully control caustic-firing temperatures.
Irrespective of caustic properties, density of sintered periclase may be quite sensitive to variations in high-firing temperature. Particularly, in the lower range of high-firing temperatures, such as 1300.degree.-1600.degree. C., resultant sintered density is generally lower than that achieved at higher temperatures. Thus, as with caustic-firing, carefully-controlled higher temperatures, accompanied by increased energy consumption, have been necessary.
Some sources of Mg(OH).sub.2, having certain impurity levels, have invariably yielded sintered periclase grain of an average grain size between 9-13.mu.. For effective refractory use, however, sintered periclase particles must have an average grain size of about 25-30.mu.. Therefore, it has heretofore been impractical to use such Mg(OH).sub.2 sources in processes for producing sintered periclase from Mg(OH).sub.2.
As an effective solution to the above-mentioned problems, the present invention promotes the growth of symmetrical single crystals of MgO, weakens the bonding of agglomerates in the caustic, and alters the morphology of the caustic. As a result of these fundamental changes in crystallinity, superior properties are obtained for both caustic magnesia and sintered periclase.
Compaction of the caustic is facilitated by powder density being independent of calcining temperature, by a reduced need for hot briquetting or lubricating aids, and by greater cohesiveness of briquettes. Higher values of green density are obtained, especially at lower calcining temperatures. Higher fired densities can be obtained and grain growth increased at lower firing temperatures. Better results are also obtained in the direct high firing of magnesium hydroxide. Additionally, the invention is inexpensive, non-toxic and does not alter the composition of sintered periclase.